Slow Carbon Cycle + Fast Carbon Cycle

Question 1

Slow Carbon Cycle

Answer 1

• Carbon stored in rocks, sea-floor sediments and fossil fuels is locked away for millions of years. The total amount of carbon circulated by this slow cycle is between ten and 100 million tonnes a year. CO2 diffuses from the atmosphere into the oceans where marine organisms, such as clams and corals, make their shells and skeletons by fixing dissolved carbon together with calcium to form calcium carbonate (CaCO3). On death, the remains of these organisms sink to the ocean floor.
• There they accumulate and over millions of years, heat and pressure convert them to carbon-rich sedimentary rocks.
  Typical residence times for carbon held in rocks are around 150 million years. Some carbon-rich sedimentary rocks, subducted into the upper mantle at tectonic plate boundaries, are vented to the atmosphere in volcanic eruptions.
• Others exposed at or near the surface by erosion and tectonic movements are attacked by chemical weathering (pages 106-07).
• Chemical weathering processes such as carbonation are the result of precipitation charged with CO, from the atmosphere, which forms a weak acid. The acid attacks carbonate minerals in rocks, releasing CO, to the atmosphere, and in dissolved form to streams, rivers and oceans.
• On land, partly decomposed organic material may be buried beneath younger sediments to form carbonaceous rocks such as coal, lignite, oil and natural gas. Like deep-ocean sediments, these fossil fuels act as carbon sinks that endure for millions of years.

Question 2

Fast Carbon Cycle

Answer 2

• Carbon circulates most rapidly between the atmosphere, the oceans, living organisms (biosphere) and soils. These transfers are between ten and 1000 times faster than those in the slow carbon cycle. Land plants and microscopic phytoplankton in the oceans are the key components of the fast cycle. Through Photosynthesis they absorb CO, from the atmosphere and combine it with water to make carbohydrates (sugars/glucose). Photosynthesis is a fundamental process and the foundation of the food chain.
• Respiration by plants and animals is the opposite process and results in the release of CO, Decomposition of dead organic material by microbial activity also returns CO, to the atmosphere.
• In the fast cycle, carbon exchange also occurs between the atmosphere and the oceans. Atmospheric CO, dissolves in ocean surface waters while the oceans ventilate CO, back to the atmosphere. Through this exchange individual carbon atoms are stored (by natural sequestration) in the oceans for, on average, about 350 years.